That was Gibbon, or a fan of Gibbon?

We'll have to start by defining "the fall of Rome." 410 CE, when it was sacked? 476, when the last Roman emperor abdicated? 1453, with the fall of Constantinople? I know people that argue Rome never fell. Obviously there are as many different theories about the fall of Rome as there are possibilities of dating the fall.

Until I hear back, let's stick with 476, as that is the traditional date.

Probably the most influential work is Peter Brown, The World of Late Antiquity. Here is a review by Tom Holland (I'm not a fan, but it's not behind a paywall). The argument basically runs such that Rome never "fell" but transitioned into something closer to a medieval state, with all kinds of innovations suitable for (their) modern time and basically trucked along. Part of this was incorporating bits of Germanic culture and absorbing an ethnically German but propagandistically (?) Roman, at least in a code-switching sort of way. I couldn't find the Theoderic coin I wanted to demonstrate this, but I found a description here:

The Roman National Museum holds the only existing contemporary medallion depicting a portrait of Theodoric – a triple solidus whose inscription reads REX THEODERICVS PIVS PRINC(eps) I(nvictus) S(emper).

So Theoderic is mixing Imperial titles (pius princeps) with what are now Germanic titles (rex). I couldn't find an image, but if I remember correctly, he's wearing Germanic clothes. The article that gave me the legend says he's wearing a mustache.

The two recent collapse theories I know are Heather and Ward-Perkins. Here is the BMCR review that treats both books.

Heather argues that for a while Rome had managed Germanic migrations fine, and so something changed to suddenly make them unmanageable. That was Attila. The Huns increased pressure on Germanic tribes to migrate to the point that they destroyed Rome's tax base and Rome's ability to cope.

Ward-Perkins has a similar theory, except it wasn't the Huns but the Vandals. Ward-Perkins focuses a lot on trade circulation, what kinds of things were being made where, when did long distance trade stop. I think its on his first page that he writes "stuff happened, bad stuff, lots of it." He also relies on the tax base, but in a little more thoughtful way than I remember Heather doing. My copies are boxed up unfortunately.

Ward-Perkins thinks the Roman Mediterranean economy operated fine through the Germanic invasions for the most part. Amphorae still get shipped, high-quality pots are still mass-produced, until the Vandals got North Africa in 429. That matters because Germans didn't have boats, couldn't attack Rome's last secure agricultural province. Once the Vandals had got a fleet, Roman economic activity plummeted. The quality of material items goes away. Life gets very bad.

There are tons of other theories, but I don't think anyone takes Christianity and moral decadence seriously anymore.

There is one more I'll give you. Huntington, "Climatic Change and Agricultural Exhaustion as Elements in the Fall of Rome" The Quarterly Journal of Economics, Vol. 31, No. 2 (Feb., 1917), pp. 173-208. Unfortunately you might need JStor access to get it.

This is the most AMAZING article I have ever read! I loved. I presented it in a graduate seminar and I think I laughed all the way through the presentation.

The general thesis is that Rome fell because of climate change. That's a remarkably modern position for someone writing in 1917, right? That's something you might hear at a conference today. So far I'm impressed.

AND he has science! He has evidence from dendrochronology that average temperatures in Italy increased both during the period of the end of the Republic, and again, after a cold spell, around the collapse of the Empire. He has a typology of theories of climate change:

Four chief climatic hypotheses have held the field. Their key words are (1) uniformity, (2) local changes, (3) progressive world-wide change in one direction, and (4) pulsatory oi irregular changes sometimes in one direction and sometimes another.

He says he needs only a change of 2 degrees F to cause climactic change to bring down governments. Alright, sounds reasonable. I'm on board. Tell me about this dendrochronology.

What's that? It's not from Italy? It's from THE CALIFORNIA REDWOOD? Alright, you're supposing the climate of California and Italy were similar in the first millennia CE. Fine, they're roughly on the same parallel. But don't you think there's a difference between being confined in a bounded, fairly small sea, and being on the coast of the Pacific? Oh, there's an answer for that.

I think I'm going to have to plug in a big quote here.

It has long been known that many of the leaders both in ancient Rome and Greece belonged to the fair Nordic race. [EDIT - I overlooked highlighting this passage] Why else do so many of the ancient painted statutes of the gods and goddesses of Greece have red or yellow hair and blue eyes? Not all the leaders, to be sure, were tall, fair Nordics; for Socrates, the greatest mind of all, was short and dark - a typical member of the Mediterranean race. Yet the fair people from the north were sufficiently aggressive and dominant to cause the favorite divinities, Zeus, Apollo, Pallas, Diana, and others to be represented as of that race. [He means Greek gods are Nordic] The climate of both Greece and Italy, however, is thought by many authorities to be too sunny for the blond Nordics. It often induces diseases of the skin and nerves, and in the long run apparently lessens the rate of reproduction. Thus in a climate like that of Italy, especially southern Italy, the agressive Nordic part of the population tends to diminish. This tendency would be less, however, under the conditions which we suppose to have prevailed three or four hundred years before Christ, and on the other hand, would increase with the changes of climate here described. When combined with the Roman prac- tice of beinging in slaves from conquered countries, it may have helped to bring about a gradual change in racial type. The race which apparently provided the majority of Rome's early leaders appears to have declined in numbers, and the decline was presumably hastened....

On the basis of the actual achievements of thousands of people under different conditions of climate, it is possible to make a map showing the amount of energy which different races would have in different parts of the world on the basis of climate alone.' This map is strikingly like a map of civilization. The resemblance of the two indicates that today the active and progressive races, those that dominate the world, are all located in climates which possess a highly stimulating quality. If we are right in thinking that the response to climate is almost the same among all races, the matter is highly significant in our interpretation of the fall of Rome. As we have already seen, the climatic changes which have apparently taken place in Italy appear to have been characterized by a decline in the variability of the weather from day to day, especially in the spring and summer. This means that three or four centuries B.C., Rome was blessed with a climate whose mean temperature was as good as that of today, and which at the same time was better than that of the present, not only for agriculture, but in its stimulating effect on human activity. It apparently possessed the sparkle and tang which our own climate in the northern United States possesses to so marked a degree.

If this is so, the change which took place between 300 and 200 B.C. and still more the gradual change between the time of Christ and the seventh century probably had an appreciable effect upon the energy and ability of the Roman people.

So, Rome collapsed because of the two conditions of a slave state and higher average temperatures drove out Rome's original Nordic population, or reduced them to the intellectual moral equivalents of Cubans and other marginalized groups? ("Strange as it may seem, the negro or Cuban in the southern part of the United States shows scarcely more adaptation to a hot climate than does the white man, while the Finn and Swede of the far north are weakened by low temperature almost as much as is the negro.")? And that the climate of the northern USA is in the sweet spot that made Rome's Nordic population so effective? Ahahahahaha!

This is the most fantastic thing I've ever read!

Moral decay as a cause for the 'fall' of Rome seems doubtful, because I see no evidence that people in the late Roman empire were any worse than their forefathers. If anything, the surviving law-codes and theological treatises would suggest that as the empire became a Christian one people became more and more concerned about morals, since we start to have records of people doing some pretty extreme things in order to demonstrate their piety, such as living on pillars (Symeon the Stylite), giving away all their wealth (St Melania the Younger), and arguing that even marriage was inferior to chastity (St Jerome). These people were exceptional individuals, but they tell us a bit about the models other Christians might aspire to in the fourth and fifth centuries.

As for fiscal irresponsibility... again, I am not convinced. The ancient economy was a very complex one and we cannot overlook the role played by other political and social factors. Can you point me to a source that says this? The Western Roman Empire in the fifth century had very little money, but that was because it lost its tax-base in Africa and had to maintain a disproportionately large army to defend itself. Not really fiscal irresponsibility in my opinion.

I'm definitely one of those people who argues that 476 wasn't particularly important and you can read my perspective on the empire's decline here, which synthesised various historians' views. Aside from Heather and Ward-Perkins, I also drew upon Guy Halsall's argument that it was internal conflict that led to the western empire's decline in power. For a long-term cultural perspective, check out my answer here, which argued that very little changed in the post-Roman world and so the 'fall' of Rome didn't matter all that much. Above all, we have to keep in mind that the Eastern Roman Empire survived the fifth century and would remain an important player in European history until its very end in 1453. If the Romans in the fifth century were so decadent and unable to manage their money, how did the Romans in Constantinople manage to weather the crisis?

Rome's problem was that the economy was wrecked and transformed by the crises in the third century AD, and then the empire suddenly found itself as the end point of one of the largest sustained population migrations in history.

In earlier times, Rome was able to assimilate populations (or at least the elites of those populations) fast enough to maintain the empire. But when the Huns pushed everyone west into an already reeling empire, the Romans simply couldn't absorb them and cope fast enough before the Huns arrived.

Once the Huns arrived, the Romans essentially lost all leverage over the tribes that had migrated into the empire.